1. Field of the Invention
This invention relates to a master for electrostatic printing and a process for electrostatic printing, and more particularly, it relates to a master for electrostatic printing of high sensitivity, high durability, and panchromatic response.
2. Description of the Prior Art
Heretofore, there are known many printing methods. Among them, electrostatic printing methods belong to a special printing field. The principle of conventional printing techniques is based on selectively adhering ink to the surface of a printing master surface in accordance with the uneven surface on the printing master or a difference of solvent affinity, and then pressing the attached ink to a paper. On the contrary, in electrostatic printing the ink is not mechanically attached to a printing master, but the ink (toner) is electrostatically attached to a printing master and then transferred to a paper.
As to printing characteristics, according to usual printing methods, the ink is attached to the printing master at a relatively stable state so that many sheets of paper can be printed at high speed, but the ink disadvantageously attaches to portions other than those to be printed. On the contrary, in electrostatic printing methods the toner attaches to a master electrostatically so that the attaching state is not sufficiently stable for conventional high speed printing usually carried out under severe conditions. Problems such as dirtying as mentioned above do not occur in electrostatic printing methods. In view of the disadvantages, electrostatic printing has not been practically used where clean printing is needed. In other words, electrostatic printing is poorer than conventional printing methods for providing many sheets of print and clear print. For example, representative conventional electrostatic printing masters are a master composed of a conductive support and an insulating image overlying the conductive support and a master composed of an insulating support and a conductive image overlying on the insulating support. The image may be produced by attaching an insulating or a conductive lacquer in a form of the image pattern to the support, or by coating a photosensitive lacquer on a support, imagewise exposing and selectively removing the exposed or unexposed portions by etching. The electrostatic printing masters having such a structure as above have various drawbacks in points of sharpness of the print and durability of the electrostatic printing master when used in a most conventional electrostatic printing process. Such a process involves sequential recycling of a charging step for forming electrostatic images by selectively retaining electric charge at image portions in case of the image portions being insulating; a developing step involving conducting the development with toners charged with a polarity opposite to that of image portions; and a transferring step for transferring the toner images to a transfer paper. For example, one drawback of the conventional electrostatic printing master has an uneven surface on which images are formed and therefore, the uneven surface is damaged by mechanical abrasion during the printing process causing irregular charging so that durability of the master is very low. Further since it is very difficult to obtain a high resolving power from such an uneven surface type master, it is also difficult technically to obtain a print having high resolution. Furthermore, it is difficult to obtain images of half tone or gradation by such an uneven surface type of master.